This invention is directed to the removal of calcium sulfate and/or calcium carbonate scale deposits from the surfaces of oil and gas wells and associated equipment and from the internal surfaces of boilers, heat exchange equipment and the like. More particularly, the invention is directed to a novel composition useful in the treatment of oil and gas wells to remove accumulated water-insoluble scale deposits from the surface of subterranean rock strata, well bore casing, tubing and associated equipment, as well as for the cleaning of boilers and heat exchange equipment.
In oil and gas wells, objectionable calcium sulfate deposits in the form of gypsum accumulate in the pores and channels of the fluid-producing strata and in the well bore equipment such as the tubing, screens, chokes and pumps. These gypsum deposits may also occur in oil field surface equipment including flow lines, separators, and emulsion treaters. The deposits are crystalline masses which adhere tightly to the formations and equipment and, as the masses build up, fluid flow is hindered and finally stopped.
This scale also causes many problems in oil and gas well treating operations, particularly when it builds up in the well tubing. Such scale deposits inhibit the flow of fluids such as oil, water and/or treating fluids through the tubing. If left unchecked, a complete blockage will result.
Removal of calcuim sulfate from wells is difficult, particularly when the scale is in a massive, dense, impermeable form. Removal of such scale is presently accomplished by several methods including scraping and chemical treatment.
One conventional method involves treatment of the scale with a scale converter which converts the scale to an acid-soluble material, followed by treatment with a mineral acid such as HCl. For example, insoluble sulfate scales are generally first reacted with a converter such as a carbonate to yield a water-insoluble--acid soluble carbonate scale which is then treated with a mineral acid.
A second conventional method employs strongly caustic solutions of chelating or sequestering agents, such as ethylenediamine tetraacetic acid or nitrilotriacetic acid. The use of the sequestering agent is conventionally a one-step operation. The sequestering agents act very slowly and require very high pH conditions to dissolve the scale and relatively long well shut-downs are necessary for the treating operation.
In a variation on these methods, a sulfate scale converting solution is used which contains a water soluble converting agent such as a bicarbonate for converting the scale to a more readily-ionizable form, and a chelating agent for dissolving and complexing the converted scale. Scale removal in this instance appears to be facilitated by the concurrent converting and complexing reactions and the action is somewhat improved over the action of the chelating agent alone. Additionally, at least under low pressure conditions, carbon dioxide evolution occurs, causing agitation and apparently giving a mechanical assist to the scale removal. As with the use of sequestering agents alone, this method requires operation under controlled pH conditions. Additionally, where the scale converter employed is a carbonate or bicarbonate, increased corrosion of the iron surfaces of the casing and associated well-bore equipment may occur by way of the well-known carbonate corrosion phenomenon.
In boilers and heat exchange equipment scale buildup of a similar nature forms on the internal surfaces, interfering with efficient heat transfer. Here the insoluble scale formation occurs by way of deposition of the calcium salts normally contained in the hard water employed in these systems. Such scale is frequently referred to as lime or lime scale, though in reality these are calcium sulfates and calcium carbonates, often in association with iron, magnesium and barium salts, frequently also including the insoluble complex hydroxides. Removal of scale is normally accomplished by mechanical means including flushing, and scale removing chemical treatments similar to those employed in oil well treatment are widely used for the purposes of removing lime scale from boilers and heat exchangers.
A scale-removing composition for effecting a rapid removal of scale deposits formed in an oil well bore and associated equipment and from the surfaces contained in boilers, heat exchangers and the like which does not require potentially hazardous and corrosive, strongly caustic solutions and which can significantly improve the effectiveness of conventional chelating and sequestering agents in removing scale is clearly needed.